It could mean several different things, and, like with so many cat behaviors, context is important.

Playfulness: you may notice your cat flicking her tail when she's playful or excited. If your cat is feeling playful, her tail will flick in a smooth motion. You've probably seen her do this when playing with a favorite toy.

Idleness: Similarly, your cat may flick her tail when she's feeling idle or content. If your cat has relaxed eyes, ears in a neutral position and a generally chill vibe while she's flicking her tail intermittently, she's probably feeling good just hanging out.

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Agitation: On the other hand, tail flicking can be a sign of agitation. If your cat is flicking her tail in bursts, it may be a sign that she's becoming annoyed. If you see other signs of agitation, like dilated pupils or a crouched body, proceed with caution.

Aggression: If your cat is holding her tail low to the ground, extended in a rigid way while she flicks it back and forth rapidly, this may be a sign that she's feeling aggressive. You might see this behavior at the vet or other places that stress your cat out. If your cat's tail is puffed up, she's extra stressed and needs some space to calm down.

Always consider the context when you see your cat flicking her tail (or, really, doing anything). Here's hoping you only get the playful kind of flicks.